1. Field of the Invention
This invention relates to a noise suppressor which employs active noise control during noisy environmental conditions.
2. Description of the Prior Art
Recently, an active noise control method has been proposed in which environmental noise is subjected to noise suppression at a listening point thereof by outputting a control sound signal from a speaker using digital signal processing technology.
A noise suppressor according to the prior art will be described below while referring to the drawings attached.
FIG. 17 is a block diagram of a conventional noise suppressor, in which 1a and 1b are microphones, 2 is an adaptive filter, 3 a FIR (Finite Impulse Response) filter, 4 a LMS (Least Mean Square) computing circuit, and 5 a speaker.
The operation of the noise suppressor arranged as above will be explained.
A noise signal detected by the microphone 1a is inputted to the adaptive filter 2 and the FIR filter 3. The noise signal thus sent to the adaptive filter 2 is adaptively controlled therethrough and sent to the speaker 5 to be reproduced. The sound signal thus reproduced interferes with a noise signal from a noise source to generate an interference sound. The interference sound thus generated is detected by the microphone 1b. The signal thus detected is sent to the LMS computing circuit 4. Here, the FIR filter 3 has a transfer function C (=C1.multidot.C2.multidot.C3) between the speaker 5 and the microphone 1b set in advance, where C1 is a transfer function of the speaker 5, C2 is a spatial transfer function between the speaker 5 and the microphone 1b and C3 is a transfer function of the microphone 1b. As a result, the LMS computing circuit 4 processes an output signal of the FIR filter 3 and a signal from the microphone 1b to update a coefficient of the adaptive filter 2 using the following equation (1) so that the output signal of the microphone 1b can be minimized; EQU w(n+1)=w(n)+.alpha.x'(n)c(n)e(n)=w(n)+.alpha.r(n)e(n) (1)
where
w(n); coefficient of the adaptive filter 2, .alpha.; step parameter, PA1 r(n); output signal of the FIR filter 3 (filtered -x signal) r(n)=x.sup.T (n)c(n), PA1 e(n); output signal of the microphone 1b, PA1 x(n); output signal of the microphone 1a, and PA1 c(n); coefficient of the FIR filter 3, T; transformed matrix.
Thus, the noise signal from the microphone 1a is adaptively processed by the adaptive filter 2 and reproduced by the speaker 5. As a result, the reproduced sound signal by the speaker 5 is canceled with the noise signal from the noise source at the microphone 1b, resulting in a reduction in noise.
Such a method that the transfer function from the speaker 5 to the microphone 1b is identified in advance using the FIR filter 3 as explained above is called a filtered -x algorithm (see, B. Widrow and S. Stearns; "Adaptive Signal Processing", Prentice-Hall, Englewood Cliffs, N.J. 1985).
With the arrangement as shown above, however, a problem has been identified in that if the noise frequency band to be suppressed has any frequency component where the noise signal at the microphone 1b and the noise signal from the noise source at the microphone 1a are insufficiently correlated with each other, it remains uncanceled even if adaptively controlled, so that the other frequency components cannot be sufficiently canceled or may be disadvantageously diverged.
In addition, a problem has been further identified in that if the noise signal detected by the microphone 1a or microphone 1b is non-white, the noise suppression quantity is varied or the convergent time is delayed depending on the frequency.
Further in addition, a problem has been also identified in that it is impossible to cancel only the noise in a specific range of the noise band in order to cancel only such a noise that is offensive to the ear based on considerations regarding characteristics of human sensations.